The present invention relates to apparatus for detecting an empty breathing gas compartment in a ventilator for a patient.
In medical applications, ventilators are widely used for patients unable to breath spontaneously. The inability to achieve spontaneous breathing may result from various lung diseases, or may be artificially produced as a part of a patient treatment process, such as anesthetizing a patient for surgery. The life of a patient unable to breath spontaneously depends on the proper functioning of the ventilator. Therefore, the reliability and safety of the ventilator are of central importance.
Ventilators are connected with patient airways through a breathing circuit. Through this circuit, respiratory gas is pushed into the patient's lungs by pressurizing the breathing circuit during an inspiratory phase of the breathing cycle. When a preset inspiration end condition is met, the ventilator switches automatically to an expiratory phase of the breathing cycle. In this stage, pressurization of the breathing circuit ceases and the existing pressure is released. The expiration of gas from the lungs occurs as a result of the elasticity of the patient's lungs. The ventilator repeats the breathing cycles continuously at a preset respiration rate throughout the treatment period.
Breathing circuits can be generally divided into two main categories: open circuits and circle, or closed, systems. Open circuits are favored in intensive care therapy where the inhaled gas is usually a mixture of oxygen and nitrogen and the ventilator directly feeds the breathing circuit with the gas. In anesthesia, the gas mixture also includes nitrous oxide and volatile anesthetic agents. Because the latter gases are expensive, closed systems are used to save the gas by circulating the same gas repeatedly to the patient with the removal of exhaled carbon dioxide and the supply of fresh gas to compensate for the gas consumed by the patient. The amount of the fresh gas needed is small compared to the ventilation volume of the breathing circuit. Therefore, when anesthesia ventilators are used, the fresh gas delivery is separate from the recirculating ventilation function. The fresh gas mixture intended for breathing is delivered through a separate gas supply line connected to the breathing circuit. Within the breathing system the fresh gas is mixed with the already existing gas in the breathing circuit to form the breathing gas for the patient.
In one of the most common approaches to anesthesia ventilation, the gas intended for patient breathing is pushed into the patient's lungs by pressurizing a container having two compartments separated by a moving barrier, such as a bellows. The breathing gas is on one side of the barrier. To pressurize the container, gas from a pressurized gas supply, such as oxygen or air, is provided to the other side of the barrier. This gas is commonly termed the driving gas. A driving gas control in the control unit for the ventilator regulates the driving gas flow into a driving gas compartment of the container causing the driving gas volume within the container to increase and correspondingly decreasing the volume of a breathing gas compartment containing the breathing gas. The breathing gas forced from the container is delivered through the breathing circuit to the patient's lungs. When the ventilator control unit instructs the driving gas control to release the gas from the driving gas compartment of the container, the gas under pressure within the patient's lungs flows to the breathing gas compartment of the container causing the breathing gas compartment volume to increase and the driving gas compartment volume to decrease.
For the operation of such a system, it is of central importance, that the moving barrier be located within the container at a position where it is freely moveable. One situation where this prerequisite is not fulfilled is upon emptying of the breathing gas volume from the breathing gas compartment of the container. In this situation the barrier is no longer moveable, e.g. a barrier of the bellows type is completely collapsed, and the ventilator is unable to ventilate the patient. This type of situation leads immediately to insufficient ventilation of the patient with disastrous consequences if not rapidly corrected. Therefore, detection of the foregoing situation is a primary safety measure.
One solution to detect an empty breathing gas compartment is described in U.S. Pat. No. 5,662,099. In this solution, the pressures of the breathing gas and of the driving gas, or alternatively, the differential pressure across the barrier, are measured and the fact that breathing gas compartment is empty is detected when the pressure difference exceeds a predetermined offset value. However, this solution fixes the breathing circuit pressure sensor position to a location that is near the barrier between the driving and breathing gas compartments of the container. Thus, a further, dedicated sensor is required if the patient circuit pressure is to be measured near the patient, or within the trachea of the patient. Obtaining pressure measurements from the latter location provides the ultimate primary information to control patient ventilation.